1. Field of the Invention
The present invention relates generally to metal hydride batteries and more specifically, relates to metal hydride batteries including a hydrogen storage capacity which provides for passive purification of a hydrogen gas stream to remove water vapor and oxygen entrained within the hydrogen gas.
2. Background Art
Metal hydride electromechanical fuel cells are in series consideration as the next generation power source for providing power to automotive and other zero emission locomotive uses. Several innovative techniques have been described for obtaining electrical power from electrochemical cells utilizing the combining reaction of hydrogen with oxygen to produce water. One classification of these techniques is between reversible and irreversible reactions.
Irreversible reactions provide a supply of hydrogen, usually in gas form, to an electrochemical cell which utilizes the combining reaction to produce an electrical potential and water as a byproduct. These types of cells utilize stored hydrogen to produce water which is then wicked or siphoned off from the system. An example of this type of electrochemical system may be found in a vehicle commonly referred to as the "Ballard bus". A variation of an irreversible system is also described in U.S. Pat. No. 4,826,741. The hydrogen is not stored as a gas under pressure, but is stored within a metal hydride material removed and separate from the electrochemical cell.
One drawback to the irreversible systems is that fresh hydrogen must be continually supplied to the system in order to continue the operation of the electrochemical cell. A means must be included for removing and disposing of the water which is a by-product of the invention. Another drawback to the irreversible systems is that the battery of cells is not rechargeable, that is, the chemical reaction producing water cannot be reversed by providing an electrical potential and thereby regenerating the constituent hydrogen and oxygen molecules for later use in electrical power production by the electrochemical cell.
Reversible electrochemical cell systems have also been described. These systems provide for a reversibility of the chemical reaction that produces water. The reverse reaction will generally utilize an electrical load having reversed polarity and being attached to the terminals. The electrical load recharges the battery by hydrolyzing water into its constituent molecules of hydrogen and oxygen. The hydrogen enters a gas phase and must then be stored for further use during the discharge mode of the battery.
Generally, nickel hydrogen batteries have relied upon a high pressure storage vessel for hydrogen gas which is used as a source of supply to produce electrical power from the electrochemical cells. Such devices require a massive, bulky and rigid outer shell which can withstand high internal pressures. High pressure gas storage has been considered necessary in order to make possible sufficient hydrogen storage capacity for continued operation of these devices. Examples of these devices are disclosed in U.S. Pat. Nos. 5,082,754, and 5,162,171.
Hydrogen storage technology has progressed to enable storage of hydrogen at lower pressures in a vessel so as to provide low pressure delivery of hydrogen gas to a nickel-hydroxide/hydrogen battery with which the storage vessel is associated. For example, a low pressure hydrogen storage system is disclosed in U.S. Pat. No. 3,850,694, issued to Dunlop et al, in U.S. Pat. No. 4,395,469 issued to Fritts, and in U.S. Pat. No. 5,250,368 issued to Golben et al.
The effective operation of metal hydride battery and storage systems at low pressures is significant because it leads directly to further refinements in the construction and use of metal hydride batteries. U.S. Pat. No. 5,419,981, invented by the inventor of the present invention, describes a further refinement in the metal hydride battery which becomes possible from the ability to use low pressure storage and low pressure hydrogen during battery operation. That refinement provides for a battery having modular construction, and for lower hydrogen pressure which allows O-ring sealing between the outer containment can portions of each module. The disclosure of U.S. Pat. No. 5,419,981 is incorporated herein by reference.
The device described in U.S. Pat. No. 5,250,368 utilizes remote hydrogen storage facilities providing a hydrogen storage vessel which is isolated from the metal hydride battery cells where the electrochemical reaction provides electrical power. An interconnecting fluid communication means, such as a pipe or other conduit, provides a reversible pathway for a hydrogen stream to flow between the storage vessel and the metal hydride battery.
Separate and isolated facilities have been considered necessary to inhibit or essentially eliminate the deterioration of various elements of a hydrogen battery/storage system which results from repeated charge/discharge of the system. The cycle includes hydrogen discharging from the storage vessel to the battery for producing electricity when an electrical load is connected. When an external electrical power source is connected, the cycle charges the system by returning the hydrogen from the battery cells to the storage vessel.
Aforementioned U.S. Pat. No. 5,250,368 describes the process by which deterioration of metal hydride material occurs, and provides a system and method for purifying the hydrogen stream in order to extend the cycle life of the battery system. The improved system and method disclosed in U.S. Pat. No. 5,250,368 and assigned together with this invention to a common assignee, actively purifies the hydrogen stream of entrained oxygen and water vapor. The description found in U.S. Pat. No. 5,250,368 is incorporated herein by reference.
U.S. Pat. No. 4,343,770 describes a segmented hydrogen storage tank containing a metal hydride hydrogen storage material and providing for a hydrogen gas stream communication between an element which utilizes hydrogen as a fuel, such as an internal combustion engine utilizing gaseous hydrogen as a fuel. A dual purpose in-line hydrogen "filter unit", is in flow communication with the hydrogen storage tank. The filter unit has a porous bed of catalyst material, such as platinum, palladium or nickel, which is capable of converting oxygen in the presence of hydrogen to water. A second section is capable of adsorbing water and water vapor from the hydrogen stream and can comprise a molecular sieve, alumina, charcoal and silica gel. The system described is usable for higher pressure for the hydrogen gas stream up to 1000 psi.
Another distinction arises within the types of reversible metal hydride fuel cells which distinction depends on how the metal hydride is utilized within the relevant system. One type of reversible battery provides a metal hydride used as an anode, and utilizing hydrogen stored within the anode for the electrochemical reaction to produce an electric charge. This type of battery is known as a nickel-metal hydride battery and the reversible electrochemical reaction proceeds as follows:
Metal Hydride (MH) Negative Electrode: EQU MH.sub.x +XOH M+H.sub.2 O+Xe PA0 Positive Electrode: EQU XNiOOH+XH.sub.2 O+Xe XNi(OH).sub.2 +XOH
An inert, hydrophilic material partially saturated with an appropriate electrolyte or a polymer membane which is susceptible to ion transmission, but is itself an electrical insulator, separates the positive and negative electrodes to avoid short circuits but nevertheless permits of the hydroxyl ion (OH.sup.-) traverse from the anode to the cathode. An effective material for the separator may be a known ionic exchange membrane, such as NAFION, manufactured by E. I. dupont de Nemours, and located in Wilmington, Del. Use of a NAFION ion exchange membrane is described in U.S. Pat. No. 4,175,165. Alternatively, a membrane having a basic ion exchange mechanism, such as fiberglass, Nylon, polypropylene or zirconia cloth, soaked in a potassium hydroxide solution, may be used for the separatore plate.
The reaction and use of an ion exchange membrane are known, and further details of the reaction may be had by reference to any of several U.S. Patents, such as U.S. Pat. No. 4,826,741, and U.S. Pat. No. 4,699,856. Of course, since the reaction is reversible, as shown by the arrows pointing in both directions, the reverse reaction charges the battery and produces atomic or molecular hydrogen which is again re-absorbed in the metal hydride anode.
A different type of electrochemical cell, known as a nickel-hydrogen battery, is used in which the anode effectively comprises a catalyst at which hydrogen gas is first split into monatomic hydrogen and ionized, and the hydrogen ion then reacts with the hydroxyl ions to drive the reaction, thereby producing water. The reaction at the positive electrode provides a hydroxyl ion which passes through a membrane separator to the negative electrode where it reacts with the hydrogen ion. The full reaction is set forth in the aforementioned Dunlop U.S. Pat. No. 3,850,694, which has been incorporated by reference.
Nickel-hydrogen batteries are different in several respects from nickel-metal hydride batteries. In the nickel-metal hydride battery example, the hydrogen is stored in solid form within the metal hydride negative electrode. The stored hydrogen is always available to run the reaction, and the only impediment to full discharge of such a battery is the lack of connection to an electrical load outside the battery. These types of batteries are subject to self-discharge, because a potential is always present at the battery terminals, and ionic particles in the environment draw down the power stored within these types of batteries.
Conversely, nickel-hydrogen batteries require a steady supply of gaseous hydrogen to provide hydrogen ions to run the electrochemical reaction. In the absence of hydrogen gas to run the reaction at the anode, the electrical potential soon dissipates after an equilibrium condition in the reversible reaction is achieved. At equilibrium, the battery may nevertheless continue to store hydrogen, and also store electrical power, if a separation between the hydrogen storage vessel and the electrochemical cells is maintained. In this way, self discharge of the battery is avoided.
Another difference between these two classes of batteries is that the nickel-metal hydride battery provides direct contact between the electrolyte separator and the metal hydride negative electrode. Introduction of the hydroxyl ions through the membrane separator permits oxygen to come into contact with the metal hydride. This is generally detrimental to the metal hydride because metal hydride materials are known to be good absorbers of oxygen as well as of hydrogen. There is a disincentive to permit oxygen absorption in the metal hydride because the oxygen impedes continued and effective absorption of hydrogen, and over a period of continued cycling, enough oxygen absorption can render the metal hydride incapable of storing sufficient hydrogen for efficient production of electric charge.
A key consideration in avoiding material deterioration or decomposition of the system elements is the elimination of impurities, such as oxygen or water vapor, from the hydrogen gas being delivered to the metal hydride storage material. Various methods have been proposed tending to inhibit or eliminate contact of oxygen or water vapor with the metal hydride electrode. U.S. Pat. No. 4,952,465 describes use of a metal hydride additive to electrodes of an alkaline energy storage device. The additive may be disposed on or between the electrodes of the alkaline energy storage device so as to absorb hydrogen and re-combine oxygen, thus reducing hydrogen and oxygen gas pressure in a sealed energy storage device and also avoiding the deterioration of the electrodes.
U.S. Pat. No. 5,128,219 also describes an electrode protection mechanism for inhibiting contact of the metal hydride, hydrogen storing negative electrode with oxygen formed during the electrolytic reaction. The mechanism includes galvanically coating metal hydride particles with a thin film of a metal, such as palladium, nickel or copper. Choosing the appropriate combination of metals having an appropriate thickness provides a "filter" which ideally is permeable to hydrogen but which inhibits passage of the larger oxygen molecules/ions from outside of the coating into contact with the encased metal hydride particles. The metal hydride particles are coated before the particles are pressed together to provide a solid metal hydride electrode.
These methods of preventing oxygen contamination of a metal hydride electrode for hydrogen storage in an electrode. However, the process for uniformly coating the individual metal hydride particles with a metal coating is expensive for the large metal hydride quantities needed for use in a metal hydride hydrogen storage battery system.
Nickel-hydrogen batteries normally provide a storage capacity for hydrogen which is separate and removed from the electrochemical cells. The storage may be in hydrogen tanks as in the aforementioned Ballard bus, or in a segmented metal hydride vessel, such as in U.S. Pat. No. 5,250,368. An advantage provided by nickel-hydrogen batteries is that contact between the hydroxyl ions and the metal hydride can be avoided by the physical separation of the metal hydride from the electrochemical cells. Nevertheless, even when isolated from each other, the electrochemical reaction in the cells produces sufficient water vapor that becomes entrained in the hydrogen stream that the water vapor reaches the metal hydride and releases the oxygen atoms which are absorbed into the metal hydride. Thus, a means is necessary to inhibit or eliminate water vapor and/or oxygen from coming into contact with the metal hydride.
U.S. Pat. No. 5,250,368 proposes not only isolating the metal hydride in a separate vessel, but including elements in an in-line piping network between the hydride storage vessel and the electrochemical cell chamber, which elements actively purify and filter out the entrained water vapor and oxygen from the hydrogen stream. This method is operational and has been demonstrated to provide a significant increase in the number of charge/discharge cycles through which the battery system may be subjected before hydrogen storage capability appreciably deteriorates. Nevertheless, the active elements of the system utilize electrical power which in smaller systems may render the electricity storing capability inefficient.
An exception to the separate or segmented storage of the hydrogen storing metal hydride material, isolated from the electrochemical cells, can be found in the aforementioned Dunlop U.S. Pat. No. 3,850,694. Dunlop proposes a thin film of Teflon using a metal hydride layer for storing hydrogen within the chamber also containing the electrochemical cells. The Teflon is described as providing a hydrophobic surface allowing hydrogen gas passage and inhibiting water vapor or KOH from reaching the metal hydride storage means. However, the device taught by Dunlop in U.S. Pat. No. 3,850,694 does not have an isolation of the hydrogen storage means from the electrochemical cells, and thus is also susceptible to self-discharge, as described above.
A method and apparatus for separation and purification of hydrogen from a gaseous mixture, comprising a thin film membrane made of heteropoly acids and salts thereof is taught in U.S. Pat. No. 4,710,278. A catalytic agent such as nickel, platinum, palladium, or alloys thereof may be disposed on either side of the thin film membrane for promotion of the dissociation and combination of the hydrogen gases on the respective sides of the membrane.
What is considered necessary to efficient and controlled electrical discharge is a metal-hydrogen battery in which the hydrogen storage means is isolated from the electrochemical cells to avoid self-discharge and also to provide control of when the charged battery begins to provide electrical power.